The FBI's top investigator in the San Bernardino terrorist attack said Friday that the husband and wife who shot and killed 14 people on Dec. 2 intended to detonate an explosive device inside the room, though the exact timetable of the plot remains unclear.

David Bowdich, the assistant director in charge of the FBI’s Los Angeles field office, said Friday that investigators now believe that Syed Rizwan Farook and his wife, Tashfeen Malik, attempted to detonate a pipe bomb hidden inside a bag at the Inland Regional Center.

What investigators still don't know is whether they planned to detonate the bomb first and then open fire on first responders, or to detonate the device as paramedics and police descended on the facility to tend to gunshot victims.

“Was the intent to detonate prior to their attack?” Bowdich asked. “We just don’t have the answer.”

During a news briefing last week, Bowdich said Farook brought a bag containing the pipe bomb into the facility when he arrived at 8:37 a.m. An FBI affidavit said the device was made of three galvanized steel pipes and smokeless powder and was attached to a remote-control toy car. The bomb was “armed and ready to detonate.”

Agents found a remote control for a toy car in the couple’s SUV, the affidavit said.

But this marks the first time officials have said they believe the couple attempted to detonate the device.

Farook and Malik drove around San Bernardino and Redlands between the time of the attack and their deaths in a gun battle with police hours later. In that three-hour, 42-minute window, the couple did not stray far from the facility, according to Bowdich. At one point, they even returned to Waterman Avenue and drove in the direction of the building.

A federal law enforcement source previously told The Times that it is possible the couple were unable to detonate the device because the remote was out of range. Sprinklers that went off after the shooting could have also interfered with the explosives, said the source, who requested anonymity to speak candidly about an active investigation.

Bowdich said it may be impossible for investigators to determine the couple’s exact plans for use of the bomb, as they have recovered no documents, schematics or other electronic data that reveal planning for the assault.

“And I will be quite frank. I am not sure we will ever know that answer,” he said Friday.

The idea that Farook wanted to attack in waves bears an eerie resemblance to his aborted plans for a terror attack on a nearby school or freeway in 2012. Court documents show that Farook and Enrique Marquez, a longtime friend who has been charged in connection with the attacks, had previously planned to open fire on a section of the 91 Freeway or at Riverside City College.

In the event of either attack, documents show, Farook wanted to first shoot civilians, then lie in wait and fire on paramedics and police as they rushed to the scene.

Though FBI agents have scoured the couple’s social media accounts and other electronic devices, they have yet to locate the hard drive from their home computer, Bowdich said. He said the hard drive is one of the crucial pieces of outstanding evidence.

Enrique Marquez Jr. was indicted last week by a federal grand jury on charges of conspiring to provide material support to terrorists,...

The man accused of providing two rifles used in the Dec. 2 mass shooting in San Bernardino pleaded not guilty to charges against him in federal court Wednesday.

Enrique Marquez Jr. was indicted last week by a federal grand jury on charges of conspiring to provide material support to terrorists,...

(Paloma Esquivel)

Bowdich also said Friday that investigators continue to explore whether Farook’s relatives, or anyone else, had prior knowledge of the attack. During meetings, survivors have asked Bowdich how Farook’s family could not have known about his plans, given the stockpile of weapons found inside the family’s Redlands home.

“The answer is, we are still working through that,” he said Friday. “This is an investigation we are being very cautious with.... We just don’t know at this point.”

Marquez, who has admitted to purchasing two of the weapons used in the shootings, has also been charged in connection with the case. The FBI, however, said Marquez was not part of the Dec. 2 plot.

The agency has so far uncovered no evidence of plans for a second attack, despite the fact that the couple had stockpiled thousands of rounds of ammunition and a dozen explosive devices inside their home.

Six weeks into the investigation, Bowdich said there is still much that may be difficult or impossible to determine about the couple’s actions.

“One of the unexplained questions we have is, ‘Why that day and why that venue?’” he said. “Did they plan secondary attacks? We just don't know. They were heavily armed.”

Last week, the FBI released a detailed timeline of Farook and Malik’s movements between the shootings and their deaths in a firefight with police hours later. By reviewing cellphone records, surveillance footage and witness accounts, the agency has managed to determined their movements for all but 18 minutes of that time frame.

“Until we know what happened in those 18 minutes, I am uncomfortable and my investigators are uncomfortable,” Bowdich said Friday. “Because you just don’t know if they met with someone, and that is disconcerting.”